This invention relates to lithium niobate elements. More particularly, the invention relates to stoichiometric lithium niobate devices such as waveguides and modulators and methods for their manufacture.
Electro-optic modulators have become one of the key components for high speed optical transmission systems. Lithium niobate single crystal materials are used in these applications because of the high electrooptic coefficient of lithium niobate materials. Single crystal lithium niobate wafers having a thickness of about 1-2 mm are generally used as substrate materials for these applications. Waveguide are written into these substrates via a variety of techniques, including but not limited to Ti thermal diffusion, Ti ion implantation, and proton exchange. Thermal diffusion of Ti ions is the most efficient and generally accepted practice. The waveguide is written to a depth of about 10 microns. The in-diffusion of Ti is performed at temperatures near 1000xc2x0 C.
The properties of the lithium niobate crystals depend on the composition of the crystals. The most widely industrially produced crystals have a composition that is referred to as the congruent composition. This congruent composition has a lithium to niobium ratio of about 48 to 52 or 0.94. The congruent composition crystals are produced by Czochralski method from lithium niobate melts at a very high crystal growth rate, typically around 8 to 10 mm/hr.
Conversely, the lithium niobate crystals of stoichiometric composition, that is, having a lithium to niobium ratio of 50 to 50 or about 1, are reported to have significantly higher electrooptic coefficients. In addition, the crystals of stoichiometric composition have higher non-linear characteristics, wider transparency range and higher resistance to optical damage. These stoichiometric crystals have to be fabricated via flux growth process, where the Li and Niobium oxides are dissolved in high temperature solvents such as K2O or other solvents, and then crystals are grown from the solution. Typically, these stoichiometric crystals can be grown at slightly lower temperatures, but the crystal growth rate drops by an order of magnitude. Due to the low growth rate and the low crystal yield obtained from the process, these crystals are prohibitively expensive. Thus, even though the stoichiometric crystals have numerous desirable properties, the high production costs prevent their use in most applications.
U.S. Pat. No. 4,725,330 (the ""330 patent) discloses a process for equilibrating lithium niobate substrates that involves heat treating the substrate in close proximity to a powder bed containing a source of lithium in a closed container. The ""330 patent suggests placing the powders to at least within 2 cm to the substrate, and in some instances burying the substrate in the powder bed. The ""330 patent further teaches that the chemical composition of the powdered bed should be the same as the desired chemical composition in the crystal, which is a Li/Nb ratio of 50/50 or 1. The small difference in crystal composition between the powder bed and the crystal composition leads to very large equilibration times of about 500 hours. One disadvantage of this process is that processing in close proximity to a powder bed will require further processing steps such as polishing and clean to remove any adhered powder to the substrate surface. In addition, the processing times of 500 hours are time consuming and undesirable for mass producing optical devices.
It would be desirable to provide improved methods for manufacturing stoichiometric lithium niobate and devices made from stoichiometric lithium niobate. In particular, it would be advantageous to provide methods that are simple and economical and that do not require extensive cleaning or polishing after adjustment of the stoichiometry of the elements. It would also be desirable to provide a method that is compatible with waveguide writing processes utilizing Ti thermal diffusion.
The invention relates to methods of producing stoichiometric lithium niobate crystals and devices made from these crystals. The crystals are produced by adjusting the stoichiometry of a portion of a congruent niobate crystal substrate having a congruent composition until the portion of the substrate has a stoichiometric composition. As used herein, the terminology congruent composition refers to lithium niobate crystals having a composition that contains a lithium to niobium ratio of less than 1, and more specifically crystals that have a lithium to niobium ratio of 0.94. As used herein, the terminology xe2x80x9cstoichiometric compositionxe2x80x9d refers to a lithium niobate substrate or portion thereof having a ratio of lithium to niobium that is approximately equal to 1, or in other words a 50/50 ratio of lithium to niobium.
One embodiment of the invention relates to a method of changing the stoichiometry of a lithium niobate substrate including mixing powders containing lithium and niobium and sintering the powders to provide a monolithic solid. In preferred embodiments, the composition of the sintered monolithic solid has a Li/Nb ratio greater than about 1, and more preferably the ratio of Li/Nb in the monolithic solid is greater than 1.5. The method further includes placing the monolithic solid proximate to the lithium niobate substrate and heating the monolithic solid and substrate to a temperature between 800xc2x0 C. and 1200xc2x0 C. for a period of time sufficient to change the stoichiometry in a portion of the substrate. Typically, the substrate and the monolithic substrate are separated by a distance of less than about 1 centimeter, preferably less than about 5 millimeters, and in highly preferred embodiments, the substrate and the monolith solid are separated by about 1 to 2 millimeters. Examples of powders that include lithium and niobium include, but are not limited to, lithium oxide, lithium carbonate, niobium oxide, niobium carbonate, lithium nitrate, lithium acetate, lithium chloride and mixtures thereof.
In certain embodiments of the invention, the powders are sintered at a temperature between 500xc2x0 C. and 1200xc2x0 C. for less than five hours. Preferably, the ratio of lithium to niobium in the solid is greater than the ratio of lithium to niobium in the substrate. In preferred embodiments, the ratio of lithium to niobium in the solid is greater than 1 and the ratio of lithium to niobium in the substrate is less than 1. Typically, the ratio of lithium to niobium in a lithium niobate crystal substrate having a congruent composition has a ratio of lithium to niobium of about 0.94. In certain embodiments, the substrate and the solid are heated to a temperature between 1000xc2x0 C. and 1100xc2x0 C. for less than about 30 hours, preferably less than about 25 hours and more preferably less than about 20 hours.
In certain embodiments of the invention, the substrate is a planar substrate, such as a wafer, and the stoichiometry of the substrate is changed in only a surface layer of the substrate. The surface layer of the substrate typically has a depth of less than about 200 microns. In some embodiments, the surface layer is less than about 100 microns, and in other embodiments the surface layer is less than 50 microns. In some embodiments of the invention, Ti is simultaneously diffused into the surface layer of the substrate while the substrate and the solid are being heated together.
Another embodiment of the invention relates to a method of producing a lithium niobate waveguide. This embodiment includes diffusing lithium from a monolithic solid containing lithium and niobate into a surface layer of a lithium niobate substrate having a ratio of lithium to niobium less than 1 and diffusing Ti into a surface layer of the lithium niobate substrate during the step of diffusing lithium into the surface layer of the substrate. Diffusion of lithium and titanium can be accomplished by heating the monolithic solid and the substrate to a temperature of between 1000xc2x0 C. and 1200xc2x0 C. The duration of the heating will depend on the actual temperature and the depth of diffusion desired. Preferably, the surface layer is less than about 200 microns, and after the step of diffusing lithium into the substrate, the substrate contains a ratio of lithium to niobium of 1 so that the surface layer has a stoichiometric composition.
The method of the present invention allows for the formation of lithium niobate substrates, having a Li/Nb ratio in surface layer of 1 and a Li/Nb ratio beneath the surface layer of less than 1. Such substrates can be used to manufacture optical devices including, but not limited to modulators and waveguides.
The present invention provides a relatively simple and inexpensive method of producing stoichiometric lithium niobate. By utilizing a monolithic solid to adjust the composition of a portion of a lithium niobate substrate, no additional cleaning or polishing is required to remove adhered powder particles to the surface of the substrate. In addition, the production method can be performed simultaneously with a Ti diffusion process for writing waveguides in lithium niobate substrates. Additional advantages of the invention will be set forth in the following detailed description. It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed.